Signing Out

After logging into my WP, I will have to resign in… sometimes I can do things for an hour before it logs me out but most of time its after a minute or two. This is been happening for a while now (for at least the past few upgrades so I know its not because of the version and I dont have the same problem with the same version on my other WP sites under the same host).
What can be causing this and how do I fix it?
The domain is http://www.sclubmedia.net/ and hosted by my other site http://www.uninvitedgrace.com/ which as I said above has not had the same problem.